Eric Schreiber

Design of an integrated Ka band receiver module for passive microwave imaging systems

Passive microwave (MW) remote sensing (radiometry) relies on the thermal radiation of objects having a temperature higher than 0 K within the frequency range of 1–300 GHz. The intensity of this radiation depends on the surface characteristics, the chemical and physical composition, and the temperature of the material. So it is possible to discriminate and to image objects having different material characteristics and hence different brightness temperatures compared to their surrounding. The range of applications of microwave remote sensing systems is spread out widely. For example, in Earth observation missions it is possible to estimate the salinity of oceans, the soil moisture of landscapes or to extract atmospheric parameters like the liquid water content of clouds or the oxygen content [1,4]. Due to the penetration capabilities of electromagnetic waves through dielectric materials, and the purely passive character of this kind of remote sensing technique, it nowadays is used as well in many security and reconnaissance applications. Examples here are the observation of sensitive areas or the detection of hidden objects like weapons or explosives during security checks. Presently different imaging principles for MW radiometry are in use. Most of them still perform pure mechanical scanning as well as a combination with electronic scanning by using parts of a focal plane array, for instance, as known from modern optical cameras. In principle, there are two main problems with mechanical scanning systems, on one hand the antenna aperture dimension has to be large for a given wavelength in order to get a sufficient spatial resolution. On the other hand it is important to record an image in a reasonable period of time. Most of the mechanical scanning systems are working with a rotating antenna structure. The velocity of this rotation cannot be increased arbitrarily due to inertia problems caused by the antenna size and mass. Hence, the trend is going towards fully electronic and quick beam steering or two-dimensional focal plane arrays. These systems are able to achieve high frame rates, but they still are very expensive, because they require a significantly higher number of receiver modules compared to a mechanical scanning system. Furthermore one has to handle a rising complexity by the integration of such a high number of receiver modules, all consisting of many discrete components following the antenna frontend. Also the weight is an important factor with respect to airborne/spaceborne platforms. Consequently, in order to minimize the weight and the costs, the whole receiver components have to be realized in a considerably integrated design by using MMIC (Monolithic Microwave Integrated Circuit) technology as far as possible.

Status of VESAS: a fully-electronic microwave imaging radiometer system

Present applications of microwave remote sensing systems cover a large variety. One utilisation of the frequency range from 1 - 300 GHz is the domain of security and reconnaissance. Examples are the observation of critical infrastructures or the performance of security checks on people in order to detect concealed weapons or explosives, both being frequent threats in our world of growing international terrorism. The imaging capability of concealed objects is one of the main advantages of microwave remote sensing, because of the penetration performance of electromagnetic waves through dielectric materials in this frequency domain. The main physical effects used in passive microwave sensing rely on the naturally generated thermal radiation and the physical properties of matter, the latter being surface characteristics, chemical and physical composition, and the temperature of the material. As a consequence it is possible to discriminate objects having different material characteristics like ceramic weapons or plastic explosives with respect to the human body. Considering the use of microwave imaging with respect to people scanning systems in airports, railway stations, or stadiums, it is advantageous that passively operating devices generate no exposure on the scanned objects like actively operating devices do. For frequently used security gateways it is additionally important to have a high through-put rate in order to minimize the queue time. Consequently fast imaging systems are necessary. In this regard the conceptual idea of a fully-electronic microwave imaging radiometer system is introduced. The two-dimensional scanning mechanism is divided into a frequency scan in one direction and the method of aperture synthesis in the other. The overall goal here is to design a low-cost, fully-electronic imaging system with a frame rate of around one second at Ka band. This frequency domain around a center frequency of 37 GHz offers a well-balanced compromise between the achievable spatial resolution for a given size, and the penetration depth of the electromagnetic wave, which are conflictive requirements.

VESAS: a novel concept for fully-electronic passive MW imaging

These days passive microwave (MW) remote sensing has found many applications. For example, in Earth observation missions, it is possible to estimate the salinity of oceans, the soil moisture of landscapes, or to extract atmospheric parameters like the liquid water content of clouds [1, 2, 3]. Due to the penetration capabilities of microwaves through many dielectric materials, and the purely passive character of this kind of remote sensing, this technique nowadays is considered as well in many security and reconnaissance applications (e.g. observation of sensitive areas, detection of concealed objects, trough-wall imaging, etc.). Presently different imaging principles for MW radiometry are possible. Most of them still are based on pure mechanical scanning or they combine this with electronic scanning by using parts of a focal plane array [4]. Due to many advantages, the technological trend is going towards fully-electronic beam steering or two-dimensional focal plane arrays. These systems are able to achieve high frame rates, but they are still very expensive because of a significantly higher number of receiver modules, compared to a mechanical scanning system. In our approach a novel concept for a Ka band fully-electronic wide swath MW imaging radiometer system is introduced [5]. It is based on a combination of beam steering by frequency shift for one scanning direction using a slotted-waveguide antenna, and the application of aperture synthesis in the other. In the following a proof of concept is outlined using a two-element interferometer system called VESAS (Voll elektronischer Scanner mit Apertursynthese) demonstrator. The advantage of using the aperture synthesis technique is the possibility to implement minimal redundant sparse arrays without a degradation of the antenna pattern. In combination with the beam steering by frequency shift, one requires a one dimensional receiver/antenna array for a two dimensional imaging, hence a low-cost, fully-electronic wide swath microwave radiometer system with high frame rates is feasible. In the following a proof of concept is outlined by presenting different MW imaging measurement results, using this kind of imaging principle.

First design investigations on a fully-electronic microwave imaging radiometer system

Present applications of microwave remote sensing systems are spread out widely. One topic for using the frequency range1 - 300 GHz is the domain of security and reconnaissance. Examples are, the observation of sensitive areas or the performance of personal security checks in order to find hidden weapons or explosives, both being an important mean in our world of a growing international terrorism. The imaging capability of hidden objects is one of the main advantages of microwave remote sensing, because of the given penetration of electromagnetic waves through dielectric materials in this frequency domain. The physical effect used in passive microwave sensing relies on the thermal radiation of objects above a temperature of 0 K. The intensity of this radiation depends on the surface characteristics, the chemical and physical composition, and the temperature of the material. So it is possible to discriminate objects having different material characteristics like ceramic weapons or plastic explosives with respect to the human body. Considering the use of a people scanning system in airports, railway stations, or stadiums, it is important that passive microwave imaging devices have no exposure on the scanned object, like active devices do. Especially for highly frequent passed security gateways it is important to have a high through-put rate in order to minimize the queue time. Consequently fast imaging systems are necessary. In the following the conceptual idea of a fully-electronic microwave imaging radiometer system is introduced.

Imaging of satellites in space (IoSiS): challenges in image processing of ground-based high-resolution ISAR data

The Microwaves and Radar Institute of German Aerospace Center (DLR) is currently developing an experimental radar system called IoSiS (Imaging of Satellites in Space), for the purpose of gathering high-resolution radar images of objects in a low earth orbit. The basic purpose of the instrument is the analysis of satellite structures for detection of possible mechanical damages or irregularities generated by space debris, for example. Furthermore investigations on unknown objects or satellites can be performed. Based on inverse synthetic aperture radar (ISAR) geometry, the ground-based pulse radar creates high-resolution range profiles over a certain azimuth angle by tracking the space object or satellite using a steerable antenna system. The guided tracking of objects during overpass, whose trajectory is sufficiently known, allows wide azimuth observation angles. Thus high azimuth resolution in the order of the range resolution can be achieved. The range resolution is given by the radar bandwidth of up to 4.4 GHz resulting in a theoretical range resolution of up to few centimeters. Considering very high-resolution imaging of objects in a low earth orbit, several error sources have to be taken into account in order to achieve desired image quality. This paper outlines main challenges of the imaging process and discusses main error sources and its influence on the ISAR image. Such error sources, like atmospheric distortion or inaccurate orbit information, primarily generate severe blurring of the ISAR image making proper focusing very challenging. Therefore, proper error correction is essential.

Investigations on the detection of thin wires using MIMO SAR

The detection of improvised explosive devices (IED) is still a challenging task. Important components of these IEDs are often thin pressure plate structures which connect the activator with the explosive device by wires. The detection of wires could therefore be useful to detect the IEDs, since the detection of the explosives itself by identifying specific characteristics is impossible for many sensor types, and quite expensive and time consuming for few being able to perform this task. In this paper investigations on the detection of thin wires using Multiple Input Multiple Output (MIMO) synthetic aperture radar (SAR) are discussed.

IoSiS: a radar system for imaging of satellites in space

Space debris nowadays is one of the main threats for satellite systems especially in low earth orbit (LEO). More than 700,000 debris objects with potential to destroy or damage a satellite are estimated. The effects of an impact often are not identifiable directly from ground. High-resolution radar images are helpful in analyzing a possible damage. Therefor DLR is currently developing a radar system called IoSiS (Imaging of Satellites in Space), being based on an existing steering antenna structure and our multi-purpose high-performance radar system GigaRad for experimental investigations. GigaRad is a multi-channel system operating at X band and using a bandwidth of up to 4.4 GHz in the IoSiS configuration, providing fully separated transmit (TX) and receive (RX) channels, and separated antennas. For the observation of small satellites or space debris a highpower traveling-wave-tube amplifier (TWTA) is mounted close to the TX antenna feed. For the experimental phase IoSiS uses a 9 m TX and a 1 m RX antenna mounted on a common steerable positioner. High-resolution radar images are obtained by using Inverse Synthetic Aperture Radar (ISAR) techniques. The guided tracking of known objects during overpass allows here wide azimuth observation angles. Thus high azimuth resolution comparable to the range resolution can be achieved. This paper outlines technical main characteristics of the IoSiS radar system including the basic setup of the antenna, the radar instrument with the RF error correction, and the measurement strategy. Also a short description about a simulation tool for the whole instrument and expected images is shown.

Improved characterization of scenes with a combination of MMW radar and radiometer information

For security related applications MMW radar and radiometer systems in remote sensing or stand-off configurations are well established techniques. The range of development stages extends from experimental to commercial systems on the civil and military market. Typical examples are systems for personnel screening at airports for concealed object detection under clothing, enhanced vision or landing aid for helicopter and vehicle based systems for suspicious object or IED detection along roads. Due to the physical principle of active (radar) and passive (radiometer) MMW measurement techniques the appearance of single objects and thus the complete scenario is rather different for radar and radiometer images. A reasonable combination of both measurement techniques could lead to enhanced object information. However, some technical requirements should be taken into account. The imaging geometry for both sensors should be nearly identical, the geometrical resolution and the wavelength should be similar and at best the imaging process should be carried out simultaneously. Therefore theoretical and experimental investigations on a suitable combination of MMW radar and radiometer information have been conducted. First experiments in 2016 have been done with an imaging linescanner based on a cylindrical imaging geometry [1]. It combines a horizontal line scan in azimuth with a linear motion in vertical direction for the second image dimension. The main drawback of the system is the limited number of pixel in vertical dimension at a certain distance. Nevertheless the near range imaging results where promising. Therefore the combination of radar and radiometer sensor was assembled on the DLR wide-field-of-view linescanner ABOSCA which is based on a spherical imaging geometry [2]. A comparison of both imaging systems is discussed. The investigations concentrate on rather basic scenarios with canonical targets like flat plates, spheres, corner reflectors and cylinders. First experimental measurement results with the ABOSCA linescanner are shown.

Detection of landmines and UXO using advanced synthetic aperture radar technology

A main problem of effective landmine and UXO decontamination is efficient and reliable detection and localization of suspicious objects in reasonable time. This requirement demands for fast sensors investigating large areas with sufficient spatial resolution and sensitivity. Ground penetrating radar (GPR) is a suitable tool and is considered as a complementing sensor since nearly two decades. However, most GPRs operate in very close distance to ground in a rather punctual method of operation. In contrast, synthetic aperture radar (SAR) is a technique allowing fast and laminar stand-off investigation of an area. TIRAMI-SAR is imaging radar at lower microwaves for fast close-in detection of buried and unburied objects on a larger area. This allows efficient confirmation of a threat by investigating such regions of detection by other sensors. For proper object detection sufficient spatial resolution is required. Hence the SAR principle is applied. SAR for landmine/UXO detection can be applied by side-looking radar moved on safe ground along the area of interest, being typically the un-safe ground. Additionally, reliable detection of buried and unburied objects requires sufficient suppression of background clutter. For that purpose TIRAMI-SAR is using several antennas in multi-static configuration and wave polarization together with advanced SAR processing. The advantages and necessity of a multi-static antenna configuration for this kind of GPR approach is illustrated in the paper.

On the use of passive microwave remote sensing by airborne platforms

The steady increase of air traffic and the operation of aircraft in almost all environmental conditions demand for advanced sensor concepts for safe and efficient flight service. Furthermore the birds perspective on the Earth allows manifold use of airborne imaging for civilian and military Earth observation. Passive microwave remote sensing is an attractive tool for exploring our environment, offering benefits from optical or infrared frequencies like simple interpretation of images, and those of microwaves like penetration through many obstacles for enhanced vision. Microwave radiometry is a measurement technique based on sampling the naturally generated microwave emission of matter having certain thermal energy content, i.e. a temperature being larger than the absolute zero. Hence, the operation can be executed covert due to the absence of a transmitter, and a radiometric sensor generates no additional electronic pollution, being of importance for avoiding radio frequency interference with other devices and avoiding burden on the anyway tight resources in available frequency bands. Modern microwave technology, in parallel to advanced imaging concepts, offers furthermore real-time operation and compact low-weight sensor design, both being main drivers in sensor construction for airborne vehicles. The paper addresses fundamental relations for two-dimensional imaging in various applications. Basic imaging approaches are outlined and discussed with respect to performance and expense. Various imaging examples of different applications are shown and discussed.